ap-4-12.dvi Acta Polytechnica Vol. 52 No. 4/2012 Concepts of Emission Reduction in Fluidized Bed Combustion of Biomass Amon Purgar1, Franz Winter1 1 Vienna University of Technology, Institute of Chemical Engineering, Getreidemarkt 9/166, 1060 Vienna, Austria Correspondence to: amon.purgar@tuwien.ac.at Abstract A status report on fluidized bed technology in Austria is under preparation, in response to the Fluidized Bed Conversion multi-lateral technology initiative of the International Energy Agency. This status report focuses on the current operation of fluidized bed combustors. Combustors have been installed in the following industrial sectors: pulp and paper, biomass heat and power plants, waste-to-energy plants, and communal sewage sludge treatment plants. There are also some small demonstration plants. These plants all have in common that they treat renewable fuel types. In many cases, only bio-fuels are treated. Besides the ability to burn a wide range of low-grade and difficult fuels, fluidized bed combustors have the advantages of low NOX emissions and the possibility of in-process capture of SO2. Various emission reduction concepts for fluidized bed combustors that are typical for their industrial sector are discussed. The discussion of these concepts focuses on NOX, SO2 and dust. Keywords: fluidized bed combustion, emission reduction systems. 1 Fluidized bed combustion The history of fluidized bed conversion is considered to have started in about 1920. A name linked to the development of fluidized bed conversion is Fritz Winkler, who conducted flue gas into the bottom of a vessel containing coke particles. When the volume flow of the flue gas increased, the phenomenon of fluidization could be observed. The bulk coke in- creased in volume, and Winkler observed that the motion of the coke particles was similar to that of a boiling liquid. This application can be described as fluidized bed gasification. He patented his find- ings in 1922, and continued building and investigat- ing fluidized bed applications. The first boom in the commercial use of fluidized bed conversion was in the 1930s and 1940s. The reason for this boom is easy to explain: air blowers became commercially available at that time. Further information about the history of fluidized bed conversion can be found in [1]. It should also be mentioned that, at least in Austria, there was also a big increase in fluidized bed com- bustion applications between 1980 and 1993 in the pulp and paper industry. Another increase in flu- idized bed combustion technology in the waste-to- energy industry began in 2000, and is still going on. The reasons for these booms and their relationship to flue gas cleaning will be discussed below. There are two main concepts: bubbling and cir- culating fluidized bed combustors. The two concepts are illustrated in Figure 1. Fluidized bed combustors mainly consist of a vessel containing a gas distribu- tor, the bulk bed material and the freeboard. The gas distributor, overlaid with the bed material, leads the fluidization air into the vessel. It flows through the bed material, and fluidization takes place. Gas distributors can be open or closed. If a closed gas distributor is built, all the bed material is above the distributor. If the distributor is open, the bed mate- rial is situated around the distributor. The bulk bed material consists mainly of inert sand, in most cases silica or dolomite. At standstill the vessel is not entirely filled with bed material. It is filled to a height considering expansion due to flu- idization. The empty space at the top of the vessel is called the freeboard. If the combustor is not equipped with an air staging system, the fluidization air is the entire combustion air. If there is an air staging sys- tem, the fluidization air is the primary air, possibly mixed with recirculated flue gas, and the secondary air is injected above the fluidized bed. The fluidized bed is heated to a certain temperature before the fuel supply is started. For this purpose, most fluidized bed combustors are equipped with a gas burner for the start up. Once the fluidized bed material is at the required temperature, the fuel is injected and due to the horizontal and vertical movement of the bed material the fuel is well mixed into it. The excellent mixing behavior and the high heat capacity of the sand, which acts as a mobile heat tank, ensure an even temperature and even fuel distribution in the combustion chamber. In addition, the overall tem- 103 Acta Polytechnica Vol. 52 No. 4/2012 Figure 1: Basic functionality of a fluidized bed combustor. Left: A bubbling fluidized bed combustor. Right: A circulating fluidized bed combustor perature in the combustion chamber is very insen- sitive to fuel quality fluctuations over time. These stable temperature conditions and the possibility of in process capture of SO2, when limestone is used as an additive, are the main advantages over grate furnaces and pulverized combustors. [4] Besides the enhanced constructional effort, flu- idized bed combustion technology has two main limi- tations. Depending on the ash composition, the max- imum temperature in the fluidized bed is limited. When the ash melting point is reached there is a possibility of agglomeration within the bed material, which can reduce or stop fluidization. In addition, the superficial velocity within the reactor, depending on the fluidization air flow and the cross sectional area, must be above the minimum fluidization veloc- ity, that ensures fluidization, and below the terminal velocity, which is the minimum velocity in the pneu- matic transport regime. [1,4] 2 Fluidized bed combustors in Austria A status report on fluidized bed technology in Aus- tria is under preparation, in response to the Fluidized Bed Conversion multi-lateral technology initiative of the International Energy Agency. This status report focuses on the current operation of fluidized bed com- bustors. Besides two demonstration plants and other fluidized bed conversion plants, 23 fluidized bed com- bustors with a thermal capacity of more than 1 MW were found and investigated. The 23 combustors were categorized into the following four industrial sectors, how, is described in the following enumer- ation: • Pulp and Paper. Combustors which supply a pulp and paper plant with energy and do not utilize municipal wastes. • Waste-to-Energy Industry. Combustors that utilize municipal wastes. • Biomass Heat and Power plants. Combustors that utilize only renewable fuels and are not con- nected to the pulp and paper industry. • Treatment of Communal Sewage Sludge. Com- bustors utilizing only communal sewage sludge. Figure 2: Total thermal capacity of fluidized bed combustors installed in the pulp & paper (PP), waste-to-energy (WTE), biomass heating and power plants (BHP), and treatment of communal sewage sludge (TCSS) industrial sectors 104 Acta Polytechnica Vol. 52 No. 4/2012 Figure 3: Thermal capacity of fluidized bed combustors installed in the pulp & paper (PP), waste-to-energy (WTE), biomass heat and power plants (BHP) and treatment of communal sewage sludge (TCSS) industrial sectors, over time Table 1: Hourly and daily emission standards for the investigated plants in the waste-to-energy industry [5,6] Dust (mg/m3) Corg (mg/m 3) SO2 (mg/m 3) NOX (mg/m 3) CO (mg/m3) minimum 5/5 8/8 20/20 60/55 50/50 maximum 10/10 10/10 50/50 100/70 100/50 average 7.75/7.75 8.5/8.5 40/37.5 75/66.3 75/50 3 The influence of flue gas cleaning Pulp & Paper Industry: Figure 3 shows that most of the fluidized bed com- bustors in the pulp and paper industry were installed between 1983 and 1986. Widely-used fuels are coal, biomass and fibrous rejects of the pulp and paper in- dustry. Gas as a fuel is used only for starting up the combustors. The main purpose of these boilers is to cover the main load of the energy demand of a pulp and paper plant. The legal framework at that time required these boilers to be equipped with electro- static precipitators or baghouse filters and, depend- ing on the fuels that were used, it was required to be able to add bulk limestone to the combustion cham- ber for in-process capture of SO2. Over time, some of the boilers have additionally been equipped with a selective non-catalytic flue gas cleaning system or a dry flue gas cleaning system. [3,6] Waste Incineration Industry: At the end of 2011, there were seven fluidized bed combustors in this sector. Together they have a ca- pacity of 321 MW. Four of these boilers, with a total thermal capacity of 268 MW, have been investigated closely. Table 1 shows that there is a strict legal framework in the waste-to-energy industry. In order to handle those strict standards, all the investigated plants use a similar setup of flue gas treatment systems, see Figure 4. Due to tighten- ing of the standards in recent years, this elaborate flue gas cleaning setup became necessary both for fluidized bed combustors and for grate furnaces. It should be mentioned here that standards are in- creasingly being set for shorter averaged sample time periods. This means that the flue gas clean- ing systems are designed to handle emission peaks. For this reason, the advantage of the stable op- erating conditions of fluidized bed combustors has become crucial when deciding between grate fur- naces and fluidized bed boilers. In 2006, there were three grate furnaces in Austria, with a total capacity of 87 tons of waste per hour, and three fluidized bed combustors, with a total capacity of 99 tons of waste per hour, in the process of plan- ning [2,6]. 105 Acta Polytechnica Vol. 52 No. 4/2012 Figure 4: Basic setup of the flue gas cleaning system in the waste-to-energy industry. A) fluidized bed combus- tor, B) gravitation and/or centrifugal separators, C) dry flue gas cleaning, D) baghouse filter or electrostatic precipitator, E) wet scrubbers, F) selective catalytic reduction (SCR) Figure 5: Basic setup of the flue gas cleaning system for biomass heat and power plants. A) fluidized bed combustor, B) selective non catalytic reduction, C) gravitation separators, D) selective catalytic reduction in high dust switching, E) dry flue gas cleaning system, F) baghouse filter Treatment of Communal Sewage Sludge: At the end of 2011, there were five boilers that exclu- sively utilize communal sewage sludge. Two of them have a thermal capacity below 2 MW and are not dis- cussed in this work. The other three combustors are structurally identical, and are all located in the same place. They have a thermal capacity of 20 MW each, and have basically the same flue gas cleaning systems as those sketched in Figure 4. A notable difference is that there is a fixed bed activated carbon absorber between the wet scrubbers and the SCR. In addition, no dry flue gas cleaning system is installed [6]. 106 Acta Polytechnica Vol. 52 No. 4/2012 Biomass Heating Plants: Three fluidized bed combustors were put into oper- ation in 2005 and 2006. Together they have a ther- mal capacity of 163 MW. Two of these boilers, with a thermal capacity of 116 MW, have been investi- gated closely. The two investigated biomass heating plants have almost the same flue gas treatment sys- tem, see Figure 5, except that one of them also has selective catalytic reduction in high-dust switching. An obvious difference from the boilers in the waste- to-energy industry is that there are no wet scrub- bers. This is because of the low sulfur content of the biomass. [6,7] 4 Summary In Austria, fluidized bed combustors are mainly used in the pulp and paper industry, in waste-to-energy plants, in biomass heat and power plants, and in communal sewage sludge treatment. Each of these industrial sectors uses a typical fuel mixture. A spe- cific flue gas cleaning system setup is installed for the typical fuel mixture that is used. In the pulp and paper industry, mainly baghouse filters or electrostatic precipitators are used. Some combustors also have a selective non-catalytic reduc- tion system (SNCR) and a dry flue gas cleaning sys- tem. In the waste-to-energy industry and in commu- nal sewage sludge treatment, the plants are equipped with an elaborate flue gas cleaning system. This system basically contains gravitation and centrifugal separators, a dry flue gas cleaning system, baghouse filters, wet scrubbers, and a selective catalytic reduc- tion system (SCR). The flue gas cleaning systems of biomass heat and power plants contain gravitation and centrifugal sep- arators, a dry flue gas cleaning system, and baghouse filters. Additionally, a selective catalytic reduction system in high-dust switching can be installed. Acknowledgement We would like to acknowledge the Austrian Federal Ministry for Transport, Innovation and Technology (http://www.bmvit.gv.at) for funding the Austrian activities within the International En- ergy Agency Fluidized Bed Conversion Agree- ment (http://www.iea-fbc.org). In addition, we would like to acknowledge the support provided by the IEA — Fluidized Bed Conversion Network (http://www.iea-fbc.net). References [1] Winter, F., Szentannai, P.: IEA Fluidized Bed Conversion Programme, Status Report 2010, Österreichisches Bundesministerium für Verkehr, Innovation und Technologie, 2010, Vienna. [2] Böhmer, S., Kügler, I.: Abfallverbrennung in Österreich Statusbericht 2006. Umweltbundesamt GmbH, 2007, Vienna. ISBN 3-85457-911-X. [3] Stubenvoll, J., Holzerbauer, R.: Technische Maß- nahmen zur Minderung der Staub- und NOx- Emissionen bei Wirbelschicht- und Laugenver- brennungskesseln. Umweltbundesamt GmbH, 2007, Vienna. ISBN 3-85457-837-7. [4] Zbigniew, B.: Fluidized Beds, Handbook of Com- bustion Vol. 4: Solid Fuels. WILEY-VCH Verlag GmbH & Co. KgaA, 2010, Weinheim, p. 399–433. ISBN 978-3-527-32449-1. [5] Amon, M., Grech, H.: Bericht des Bun- desministers für Land- und Forstwirtschaft, Um- welt- und Wasserwirtschaft über Verbrennungs- und Mitverbrennungsanlagen gemäss §18 AVV, Berichtszeitraum 2009, Bundesministerium für Land- und Forstwirtschaft, Umwelt- und Wasser- wirtschaft. 2010, Vienna. [6] Provided Information from the IEA-Fluidized Bed Conversion Network Austria. [7] Selcuk, N., Gogebakan, Z.: Co-Firing Biomass with Coal in Fluidized Bed Combustion Systems, Handbook of Combustion Vol. 4: Solid Fuels. WILEY-VCH Verlag GmbH & Co. KgaA, 2010, Weinheim, p. 557–608. ISBN 978-3-527-32449-1. 107